1. Field of the Invention
This invention relates generally to a method and apparatus for stall detection in respect of a motor and, more particularly but not necessarily exclusively, to a method and apparatus for stall detection in respect of a drive motor of a power-driven vehicle such as a motorized wheelchair.
Motorized wheelchairs generally include left and right drive wheels driven by a motor controller via corresponding left and right drive motors. In general, during operation, electric motors generate heat from the current flow through resistive windings. This heat energy, within the enclosed environment of a motor, can quickly raise both the internal general temperature and the spot temperature well above the rated temperature and, therefore, cause damage. This damage can be in the form of shorted or open windings, degradation of the motor assembly or even fire. Thus, the motor controller is required to include a mechanism for limiting the temperature rise to within predetermined operating limits.
In the case of a motor having multiple windings, when the motor is rotating at speed, the heat energy is applied to all windings, thereby spreading the total lost energy over a relatively large area. However, during stall conditions when the motor is stationary, all the heat energy is directed across one or two windings, which causes those individual windings to heat up far more rapidly than they would during normal operating conditions, which in turn makes them much more susceptible to failure. Thus, it is highly desirable for the temperature limiting mechanism of the motor controller to include means for detecting that the motor has stalled.
2. Description of the Prior Art
Prior art mechanisms for stall detection use direct measurement of the speed of rotation of the motor. In the event that maximum current and power are being drawn but the motor is determined to be stationary, a stall condition is indicated. Japanese patent application no. JP2004343998 describes a motor controller including a speed detection circuit for generating a signal indicating the rotational speed of the motor. Means are provided for isolating the motor from the power source in the event that a stall condition is determined based on the signal from the speed detection circuit.
It is an object of the present invention to provide a stall detection mechanism for a motor controller that does not require the use of additional, relatively expensive hardware sensors for directly measuring the rotational speed of the motor.